Hydrocarbon liquid sensor probes have traditionally been installed in the interstitial space of double-wall tanks and pipes, sumps, fuel hydrant pits, berms encircling above-ground tanks, groundwater monitoring wells and other locations where hydrocarbon leaks or spills may occur and accumulate. These hydrocarbons are typically fuels, solvents and chemical reagents. Such sensors are typically coupled with an alarm system that will sound an alarm upon the sensor's detection of the hydrocarbon in the immediate environment.
Some known hydrocarbon sensors generally operate on the principle of an electrical circuit, namely the sensor's circuit remains open while no hydrocarbons are detected and the sensor's circuit closes upon the detection of hydrocarbons. Specifically, such hydrocarbon sensors are typically formed of cooperative conductors forming an open circuit. Hydrocarbons cause a change in the electrical status of the conductors, thereby closing the circuit. Accordingly, the leaking hydrocarbon is detected by a signal being generated as a result of the completed circuit. In particular, such hydrocarbon sensors have been known to operate on the principle that a silicone and conductive particle matrix will swell when exposed to hydrocarbons, changing the resistance of the matrix. In these hydrocarbon sensors, the sensor operates like a switch.
One example of such a hydrocarbon sensor is described in U.S. Pat. No. 4,926,165 wherein the sensor contains a swellable material which when engorged, causes an electrical connection between two conductors. U.S. Pat. No. 4,972,179 employs a three conductor configuration to form a Wheatstone bridge circuit when the second and third conductors are short circuited by the presence of a liquid.
Another example of a hydrocarbon sensor is the one described in U.S. Pat. No. 4,855,706. That patent is directed to an electrical sensor and sensor material for detecting the presence of an organic liquid by contact that is not sensitive to the presence of organic vapors from the liquid. The sensor material includes a relatively large concentration of electrically conductive particles within a swellable matrix. U.S. Pat. No. 5,514,338 also is directed to a device for sensing liquid hydrocarbons. In U.S. Pat. No. 5,514,338, a ribbon-like sensor is employed, which has an inner, silicone rubber layer and an outer, silicone rubber layer and which has two broad faces and two narrow edges. The silicone rubber layers are capable of absorbing and swelling in the presence of a liquid hydrocarbon.
While all of these hydrocarbon sensors have met with a degree of success, they also have several deficiencies. For example, known hydrocarbon sensors cannot typically be reset after exposure to non-volatile fuels such as diesel fuel, jet fuel, crude oil and #6 fuel oil. Additionally, to the extent they can be reset, there are typically a finite number of exposures before the expansion/contraction cycle of the silicone matrix loosens the bond between the thin matrix film and the substrate therein. As such, there is a limit to these sensors' use. Many installations require annual activation tests to verify the sensor is functional; this reduces the life/reset ability of the sensor. Further, prior silicone sensor elements with conductive material therein are fragile, particularly when they become saturated with hydrocarbon liquid. This can result in a delaminating of the silicone from the associated substrate. Moreover, long-term exposure to water can damage the fragile sensor element or the substrate to which the sensor element is attached.
The present invention is provided to solve the problems discussed above and other problems, and to provide advantages and aspects not provided by prior sensors of this type. A full discussion of the features and advantages of the present invention is deferred to the following detailed description, which proceeds with reference to the accompanying drawings.